The present invention relates generally to input/output circuits, and more specifically to the protection of output circuits from over-voltage conditions.
In order to save power, the trend in integrated circuits has been to reduce their operating voltages. This reduction in power supply voltages has also been necessitated by the lower drain-to-source voltage of new, smaller geometry devices. For example, many circuits now operate at 1.5 or 1.8 volts, and the expectation is that these voltages will continue to be reduced in the future.
Not all integrated circuits operate at these lower voltages however. That is, many devices have been designed to operate at 2.5, 3.3 volts, or at even higher voltages. This means that some chips operating at the lower 1.5 or 1.8 volts should have outputs that are able to drive voltages in the higher supply range, and that they need to accept higher input voltages. For example, a device operating with core or internal voltage supplies of 1.5 volts and ground may need to send and receive signals that switch between 3.3 volts and ground.
This arrangement leads to several possible over-voltage problems for output drivers. For example, when an I/O cell is configured as an output and the supply voltage exceeds the drain-to-source breakdown voltage of the output driving devices, the output devices breakdown resulting in excessive currents. Also, when an I/O cell is configured as an input (or as a tri-stated output coupled to a tri-state bus) and the input voltage exceeds the supply voltage by more than a diode drop, the parasitic drain-to-bulk diode of a p-channel output device begins to conduct current, if the bulk is tied to the supply voltage. Further, in the receive mode, if the received voltage exceeds the drain-to-source breakdown voltage of an n-channel pull down device, that device may begin to conduct excess currents.
Thus what is needed are circuits, methods, and apparatus for protecting the devices in an output stage from over-voltage conditions such including those from excessive supply and received input voltages.